In order to achieve low NOx levels that are required by regulation, gas turbine combustion designs rely on lean, premixed combustor designs to reduce flame temperatures. Typically, in order to achieve the best mixing possible, fuel is injected through a plurality of orifices situated so as to inject the fuel into the incoming combustion air. These injection orifices are joined by manifolds internal to the burner structure. A similar approach is also used for liquid fuels such that a large number of injection points interconnect through an internal manifold. Liquid fuel is delivered to the internal manifold via a single feed tube. Combustion air temperature in a gas turbine is typically between 400-500° C., while the liquid fuel temperature is typically less than 50° C. Hence, during liquid fuel operation, thermal stresses within the burner structure are typically quite high, especially in the region of the fuel feed tube. The problem of high stresses in the region of the fuel feed tube is typically exacerbated by the presence of the gas manifolds, which further increase stress. Quite often, the life limiting condition of the burner is related to the operational condition where oil operation is initiated by purging the oil passages with water.
In order to account for the high combustion temperature, thermally sensitive fuel feed conduits have been developed that have outer gaps. However, the gaps often fill with liquid fuel, which has a high conductivity, thereby reducing the insulation capacity of the fuel feed conduits. Additionally, the fuel that enters the gaps is very susceptible to coking, which is the formation of carbon particles. Coking can cause clogging of the fuel injection nozzles if the particles are carried downstream. Thus, a need exists for an improved insulation system for fuel feed conduits of gas turbine engines.